a. Field of the Invention
This invention pertains to optical proximity sensors with optical fibers used for measuring distance to a target which has a narrower diameter than that of the fiber optic bundles, and more particularly to a system incorporating such sensors and having automatic reflectivity compensation.
b. Description of the Invention
Optical proximity sensors are known in which a distance to a target is determined by directing light from a source through a fiber optic bundle to the target and then measuring the intensity of the light reflected by the target. Typically, the bundle comprises transmitter and receiver fibers (for transmitting light to, and receiving light from the target respectively) which are randomly distributed. This type of bundle was founnd satisfactory for measuring distances to normal targets. However, if the target is narrower than the diameter of the bundle and, furthermore, if the target is moving in a direction transversal to the light path between the bundle and the target (for example, such as rotating turbine blades) then the normal optical proximity sensors perform very poorly. To major factors which influence the performance of normal proximity sensors in such cases are poor linearity between the distance between measured and the intensity of the reflected light and the effects of with and movement of the target with respect to the bundle. While some attempts have been made to compensate for these effects by adding various offsets, these attempts resulted in only marginal improvements.